JAHN ET AL.: FOOD-SEEKING LARVAL WHITE CROAKER 



Table 3. — Density (rotifers per liter) of Trichocerca sp. as a function of time and sampling heighit. Eachi set of three numbers gives the density 

 of 100-150 M-m, 150-200 txm, and 200-300 ^.m rotifers. Height of simultaneous pump sample is given. 



Time (PST): 

 Pump height: 



Sampling 



height 



(cm) 



Time (PST): 

 Pump height: 



Sampling 



1030 

 0.5 m 



1130 

 0.5 m 



1220 

 1 m 



100- 

 150 pim 



150- 

 200 ^.m 



200- 

 300 pirn 



100- 

 150 \i.m 



150- 

 200 M-m 



200- 

 300 M-m 



100- 

 150 pLm 



150- 

 200 M.m 



1313 

 1 m 



1425 

 6.7 m 



1454 

 6.7 m 



200- 

 300 M-m 



Table 4. — Abundance (animals m 3) of cope- 

 podite and adult Corycaeus spp. in 100 [im 

 mesh samples from the fish pump. 



Height 

 (m) 



First sample 



Second sample 



6.7 



1 



0.5 



1,080 

 120 

 180 



460 

 500 



140 



mysids, and euphausid furcilia larvae — were all 

 abundant OlO m""^) in the 0.5 m, 330 ixm mesh 

 samples but with the exception of the callianassa 

 zoea mentioned above (from the gut of an 11 mm 

 larva) were not found in these white croaker lar- 

 vae. 



DISCUSSION 



The chief drawback of the pumping system 

 used was its inability to obtain a simultaneous 

 vertical profile. The sampling sequence left the 

 possibility that differences among heights might 

 be confounded by trends in time, as discussed by 

 Jahn and Lavenberg (1986). Slight time effects 

 were found among the vertical profiles of mi- 

 croplankton, increasing the suspicion that the ap- 

 parent vertical distributions of fish larvae and 

 macrozooplanktonic prey might have horizontal 

 components. To contradict the argument that 

 food-seeking did not bring postflexion larvae near 

 the bottom, one would need to invoke either an 

 afternoon increase of some two orders of magni- 



tude in copepod abundance (Table 4) or else the 

 presence of flexion and postflexion larvae 

 throughout the water column in morning and 

 midday followed by their sudden disappearance 

 in the afternoon. 



A two-order-of-magnitude change in copepod 

 species abundance over a distance of roughly 1 

 km (2 hours at 14 cm s"M is certainly possible; 

 though zooplankton structures reported from the 

 southern California continental shelf are gener- 

 ally larger than this (Star and Mullin 1981; Bar- 

 nett and Jahn 1987), there is always the possibil- 

 ity of sampling the edge of a patch. Since no such 

 edge was evident in the abundance or overall 

 composition of microplankton or of phytoplank- 

 ton, it seems unlikely that a macrozooplankton 

 change of this order occurred. Moreover, the main 

 copepod eaten, Corycaeus anglicus, is generally 

 more abundant in midwater than near the bottom 

 over the shallow shelf (A. Barnetf^), in accord 

 with its apparent distribution in this study. As to 

 a possible midwater abundance of postflexion 

 white croaker larvae, no such concentration has 

 ever been reported. In some nine vertical profiles 

 taken in daylight over a 6-d period. Brewer and 

 Kleppel (1986) took virtually all specimens >3.5 

 mm in their near-bottom sampler. White croaker 

 appears similar to another abundant sciaenid, 

 queenfish, in this regard (cf Jahn and Lavenberg 

 1986). 



4A. Barnett, Marine Ecological Consultants, 531 Encinitas 

 Blvd.. Encinitas, CA 92024, pers. commun. July 1987. 



259 



